4 Hydrology and Water Resources NIGEL ARNELL (UK) AND CHUNZHEN LIU (CHINA) Lead Authors: R. Compagnucci (Argentina), L. da Cunha (Portugal), K. Hanaki (Japan), C. Howe (USA), G. Mailu (Kenya), I. Shiklomanov (Russia), E. Stakhiv (USA) Contributing Author: P. Döll (Germany) Review Editors: A. Becker (Germany) and Jianyun Zhang (China) CONTENTS Executive Summary 19 3 4. 5 . Impacts on Wat e r Re s o u r ces and Hazards 21 2 4. 5 . 1 . In t r o d u c t i o n 21 2 4. 1 . In t r oduction and Scope 19 5 4. 5 . 2 . Impacts of Climate Change on Water Resources: AGlobal Perspective 21 3 4. 2 State of Knowledge of Climate Change 4. 5 . 3 . Catchment and System Case Studies 21 3 Impacts on Hydrology and Wat e r Re s o u rc e s : 4. 5 . 4 . Impacts of Climate Change on Pro g r ess since the Second Assessment Report 19 5 Water Resources: An Overview 21 7 4. 2 . 1 In t r o d u c t i o n 19 5 4. 2 . 2 Estimating the Impacts of Climate Change 19 5 4. 6 . Adaptation Options and 4. 2 . 3 . Increased Awareness of the Effect of Management Implications 21 8 Climatic Variability on Hydrology 4. 6 . 1 . In t r o d u c t i o n 21 8 and Water Resources 19 6 4. 6 . 2 . Water Management Options 21 9 4. 2 . 4 . Adaptation to Climate Change 4. 6 . 3 . Implications of Climate Change in the Water Sector 19 6 for Water Management Policy 22 1 4. 6 . 4 . Factors Af fecting Adaptive Capacity 22 2 4. 3 . Ef fects on the Hydrological Cycle 19 7 4. 6 . 5 . Adaptation to Climate Change 4. 3 . 1 . In t r o d u c t i o n 19 7 in the Water Sector: An Overview 22 3 4. 3 . 2 . Pr e c i p i t a t i o n 19 7 4. 3 . 3 . Ev a p o r a t i o n 19 8 4. 7 . Integration: Wat e r and Other Se c t o r s 22 4 4. 3 . 4 . Soil Moisture 19 9 4. 7 . 1 . The Nonclimate Context 22 4 4. 3 . 5 . Groundwater Recharge and Resources 19 9 4. 7 . 2 . Water and Other Related Sectors 22 4 4. 3 . 6 . River Flows 20 0 4. 7 . 2 . 1 . Ecosystems (TAR Chapter 5) 22 4 4. 3 . 6 . 1 . Trends in Observed Streamflow 20 0 4. 7 . 2 . 2 . Coastal and Marine Zones 4. 3 . 6 . 2 . Ef fects of Climate Change (T AR Chapter 6) 22 4 on River Flows 20 2 4. 7 . 2 . 3 . Settlements (TAR Chapter 7) 22 4 4. 3 . 7 . La k e s 20 4 4. 7 . 2 . 4 . Financial Services 4. 3 . 8 . Changes in Flood Frequency 20 5 (T AR Chapter 8) 22 5 4. 3 . 9 . Changes in Hydrological 4. 7 . 2 . 5 . Health (TAR Chapter 9) 22 5 Drought Frequency 20 6 4. 7 . 3 . Water and Conflict 22 5 4. 3 . 1 0 . Water Quality 20 7 4. 3 . 11 . Glaciers and Small Ice Caps 20 8 4. 8 . Science and Information Needs 22 5 4. 3 . 1 2 . River Channel Form and Stability 20 9 4. 8 . 1 . In t r o d u c t i o n 22 5 4. 3 . 1 3 . Climate Change and Climatic Var i a b i l i t y 20 9 4. 8 . 2 . Estimating Future Impacts of Climate Change 22 5 4. 4 . Ef fects on Wat e r Wit h d r a w a l s 20 9 4. 8 . 3 . Adapting to Climate Change 22 6 4. 4 . 1 . In t r o d u c t i o n 20 9 4. 4 . 2 . World Water Use 21 0 Re f e re n c e s 22 7 4. 4 . 3 . Sensitivity of Demand to Climate Change 21 1 EXECUTIVE SUMMARY • There are apparent trends in streamflow volume—both • Demand for water generally is increasing as a result of increases and decreases—in many regions. These trends p o pulation growth and economic development, but it is cannot all be definitively attributed to changes in regional falling in some countries. Climate change is unlikely to temperature or precipitation. However, widespread accelerated have a large effect on municipal and industrial demands but glacier retreat and shifts in streamflow timing in many areas may substantially affect irrigation withdrawals. from spring to winter are more likely to be associated with • The impact of climate change on water resources depends climate change. not only on changes in the volume, timing, and quality of • The effect of climate change on streamflow and groundwater streamflow and recharge but also on system characteristics, recharge varies regionally and between scenarios, largely changing pressures on the system, how the management of following projected changes in precipitation. In some parts the system evolves, and what adaptations to climate change of the world, the direction of change is consistent between are implemented. Nonclimatic changes may have a greater scenarios, although the magnitude is not. In other parts of impact on water resources than climate change. the world, the direction of change is uncertain. • Unmanaged systems are likely to be most vulnerable to • Peak streamflow is likely to move from spring to winter in c l imate change. many areas where snowfall currently is an important • Climate change challenges existing water resources c o mponent of the water balance. m a nagement practices by adding additional uncertainty. • Glacier retreat is likely to continue, and many small glaciers Integrated water resources management will enhance the may disappear. potential for adaptation to change. • Water quality is likely generally to be degraded by higher • Adaptive capacity (specifically, the ability to implement water temperature, but this may be offset regionally by integrated water resources management), however, is increased flows. Lower flows will enhance degradation of d i stributed very unevenly across the world. water quality. • Flood magnitude and frequency are likely to increase in most regions, and low flows are likely to decrease in many regions. Hydrology and Water Resources 195 4.1. Introduction and Scope practices (such as the use of agrochemicals) are altering the hydrological system, often leading to deterioration in the resource This chapter assesses our understanding of the implications of baseline. Changing demands generally are increasing pressures climate change for the hydrological cycle, water resources, and on available resources, although per capita demand is falling their management. Since the beginnings of concern over the in some countries. The objectives and procedures of water possible consequences of global warming, it has been widely management are changing too: In many countries, there is an recognized that changes in the cycling of water between land, increasing move toward “sustainable” water management and sea, and air could have very significant impacts across many increasing concern for the needs of the water environment. For sectors of the economy, society, and the environment. The example, the Dublin Statement, agreed at the International characteristics of many terrestrial ecosystems, for example, are Conference on Water and the Environment in 1992, urg e s heavily influenced by water availability and, in the case of s u stainable use of water resources, aimed at ensuring that neither instream ecosystems and wetlands, by the quantity and quality the quantity nor the quality of available resources are degraded. of water in rivers and aquifers. Water is fundamental to human Key water resources stresses now and over the next few life and many activities—most obviously agriculture but also decades (Falkenmark, 1999) relate to access to safe drinking i n d u s t r y, power generation, transportation, and waste wa t e r , water for growing food, overexploitation of water resources m a n a g ement—and the availability of clean water often is a and consequent environmental degradation, and deterioriation constraint on economic development. Consequently, there have in water quality. The magnitude and significance of these stresses been a great many studies into the potential effects of climate varies between countries. The late 1990s saw the development change on hydrology (focusing on cycling of water) and water of several global initiatives to tackle water-related problems: resources (focusing on human and environmental use of The UN Commission on Sustainable Development published water). The majority of these studies have concentrated on the “Comprehensive Assessment of the Freshwater Resources p o ssible changes in the water balance; they have looked, for of the World” (WMO, 1997), and the World Water Council example, at changes in streamflow through the year. A smaller asked the World Commission for Water to produce a vision for number of studies have looked at the impacts of these changes a “water-secure world” (Cosgrove and Rijbersman, 2000). A for water resources—such as the reliability of a water supply series of periodical reports on global water issues was initiated reservoir or the risk of flooding—and even fewer explicitly (Gleick, 1998). The impacts of climate change, and adaptation have considered possible adaptation strategies. This chapter to climate change, must be considered in the context of these summarizes key findings of research that has been conducted other pressures and changes in the water sector. and published, but it concentrates on assessing opportunities and constraints on adaptation to climate change within the water sector. This assessment is based not only on the few studies 4.2 State of Knowledge of Climate Change Impacts that have looked explicitly at climate change but also on on Hydrology and Water Resources: Progress c o nsiderable experience within different parts of the water since the Second Assessment Report s e ctor in adapting to changing circumstances in general.